The present invention relates to a method of decontaminating PCB-containing transformers, more particularly to such a method involving a cleaning process of internal surfaces using a solvent wash process.
Polychlorinated biphenyls (PCB) are synthetic chemical compounds consisting of chlorine, carbon and hydrogen. First synthesized in 1881, PCBs are relatively fire resistant, very stable, do not conduct electricity and have a very low volatility at normal temperatures. These and other properties have made them desirable components in a wide range of industrial and consumer products. Some of these same properties make PCB environmentally hazardous, especially their extreme resistance to chemical and biological breakdown by natural processes in the environment.
The use of PCB as the insulating fluid in transformers and other electrical products such as fluorescent light ballasts was discontinued in 1978. Up to this time many transformers contained PCB as the insulating liquid replacing mineral oil in applications where a transformer failure with the resulting fire could prove disastrous. Most PCB-containing transformers are located in office buildings.
PCBs are now listed as a toxic substance under the Canadian Environmental Protection Act (CEPA) and its use in new products, and its release into the environment have been prohibited under the Chlorobiphenyl Regulations of CEPA.
Liability considerations are the chief reason for the early phase out of PCB equipments which are otherwise in serviceable condition and have adequate capacity for the operating loads. The risk of accidental release or fire is very small. However, there are substantial costs associated with the spill cleanup or building cleanup following fires originating from a non-PCB source but involving PCB equipment. The vapourization of PCB by high heat generates dioxins and dibenzofurans, which are identified carcinogens. This risk is one of the reasons that many organizations plan for the early retirement of their PCB transformation equipment.
In addition to the total destruction and replacement of PCB-containing transformers, the decontamination of operational PCB transformers is normally carried out by either a series retrofill or in-situ processor method. Both these methods have major flaws in that they either generate large amounts of contaminated transformer fluid as in the series retrofill method or require long processing times as with the in-situ processor method.
There is some doubt associated with the long term benefits of decontaminating PCB transformers without removing the core/coil assembly. This is supported by Environment Canada (EC) who have recommended that even after a transformer has been drained, retro-filled and the fluid decontaminated (with a time frame of 2 to 3 years), the transformer should be tested for an additional three years if the leaching fluid is left in and an additional ten years if it is replaced with silicone fluid. During this time it will remain on Environment Canada""s list of PCB-containing equipments and must be treated and labeled as a PCB transformer. Processes that do not remove the core/coil assembly must contend with the problem of leach back. Leach back occurs because of the large amount of porous material such as insulation and wood used in the manufacture of transformer core/coil assemblies. This material by necessity is extremely dry when installed in the transformer. The estimated amount of transformer fluid absorbed by the wood and insulation is between 3% and 5% of the total amount used to fill the transformer. For a 2000 kVa transformer which would hold approximately 1500 liters, the amount absorbed would be between 45 and 75 liters. To contaminate 1500 liters of transformer oil to over 50 ppm would only take approximately 60 grams of PCB. This illustrates the difficulty in decontaminating a PCB transformer without removing the core and coil. The PCB in the core and coil will leach back into the transformer fluid for years. Even after the leach back has slowed to an acceptable level, the PCB contained in the wood has been shown to remain over the acceptable limit of 50 ppm indefinitely and at the end of the transformer life will have to be removed and stored for eventual destruction as PCB waste.
This situation will require that upon the end of the transformer life it will have to be disassembled and the interior components tested for PCB content. As it has been determined by Environment Canada that there will be components that contain PCB over the 50 ppm, these will have to be removed and sent to a licensed PCB destruction facility.
A prior art search conducted at the Canadian Patent Office revealed the following patents that disclose methods of decontaminating PCB transformers: U.S. Pat. Nos. 4,483,717 (Olmsted et al), 4,699,667 (Walsh of Westinghouse), 4,950,837 (Horneck et al of General Electric) and 4,983,222 (Green et al of Union Carbide). They do not appear very satisfactory.
There remains a need for a method of transformer decontamination that avoids the problems associated with known methods and gives consistent reliable results.
Broadly stating, the present invention provides a method of decontaminating a PCB transformer to a level below 50 ppm when the transformer is re-filled with a new non-PCB fluid, which comprises: (a) removing a transformer core/coil assembly from the PCB transformer from which PCB has been drained off; (b) cleaning all interior surfaces and remaining components of the transformer by application of a cleaning solvent; and (c) installing a new transformer core/coil assembly.
In a preferred embodiment, the process may also include between steps (b) and (c), (d) flushing interior surfaces of the cooling fins with a replacement non-PCB fluid.
According to the process of this invention, a PCB-filled transformer can be reclaimed as a PCB-free transformer by the removal of those components that cannot be adequately cleaned and the solvent washing of all interior surfaces and components. The remaining product will be permanently less than 50 ppm PCB.